Method and device for training an rf amplifier linearization device, and mobile terminal incorporating same

ABSTRACT

a method whereby a linearization training sequence is designed to be transmitted by means of a radiofrequency transmitter incorporated in a mobile terminal or a base station of a radiocommunication system comprising a fixed network. The sequence is adapted for transmitting bursts in accordance with a specific frame structure. Besides, the sequence is included in a sequence of symbols further designed to enable the transmission chain parameters to be adjusted.

The present invention relates to the linearization of radiofrequency(RF) power amplifiers. It finds applications, in particular, in the RFtransmitters of the mobile terminals of digital radiocommunicationsystems. It may also be applied in the RF transmitters of base stationsin particular during the first startup of such a station.

In current digital radiocommunication systems, one seeks to sendinformation with a maximum throughput in a given RF frequency band whichis assigned to a transmission channel (hereinbelow radio channel). To dothis, the modulations that have been used for a few years comprise aphase or frequency modulation component and an amplitude modulationcomponent.

Moreover, radio channels coexist in a determined frequency band allottedto the system. Each radio channel is subdivided into logical channels bytime division. In each time interval (or time slot), a group of symbolscalled a burst or packet is transmitted.

It is necessary to take care that, at each instant, the power leveltransmitted in each radio channel does not jam the communications in anadjacent radio channel. Thus, specifications prescribe that the powerlevel of an RF signal transmitted in a determined radio channel be, inan adjacent radio channel, less for example by 60 dB (decibels), thanthe power level of the RF signal transmitted in said determined radiochannel.

It therefore turns out to be necessary that the spectrum of the signalto be transmitted, which results in particular from the type of themodulation employed and the binary throughput, not be deformed by the RFtransmitter. In particular, it is necessary that the RF transmitterexhibit a characteristic of output power as a function of input power,which is linear.

However, the radiofrequency power amplifier (hereinafter RF amplifier)present in the RF transmitter has a characteristic that is linear at lowoutput power but nonlinear as soon as the power exceeds a certainthreshold. It is also known that the efficiency of the RF amplifier isall the better when working in a zone close to saturation, that is tosay in the nonlinear regime. Thus, the need for linearity and the needfor high efficiency (to save on battery charge) compel the use oflinearization techniques to correct the nonlinearities of the RFamplifier. Two of the techniques most commonly employed are basebandadaptive predistortion and the baseband Cartesian loop.

In the Cartesian loop technique, the signal to be transmitted isgenerated in baseband in the I and Q format. Additionally, a couplerfollowed by a demodulator makes it possible to tap off a part of the RFsignal transmitted and to transpose it to baseband (downconversion), inthe I and Q format. This baseband signal is compared with the basebandsignal to be transmitted. An error signal resulting from this comparisondrives a modulator, which provides for the transposition to theradiofrequency domain (upconversion). The output signal from themodulator is amplified by an RF amplifier which delivers the RF signaltransmitted.

In the baseband adaptive predistortion technique, the signal to betransmitted is generated in baseband, in the I and Q format, andpredistorted via a predistortion device. Then, this signal is transposedto the RF domain by virtue of an RF modulator. Next, it is amplified inan RF amplifier. A coupler followed by an RF demodulator make itpossible to tap off a part of the RF signal transmitted and to transposeit to baseband, in the I, Q format. This baseband demodulated signal isdigitized and compared with the baseband signal to be transmitted. Anadaptation of the predistortion coefficients, carried out during a phaseof training of the predistortion device, allows the demodulated I and Qformat signal to be made to converge to the I and Q format signal to betransmitted.

In both techniques, a part of the signal transmitted is tapped off atthe output of the RF amplifier so as to compare it with the signal to betransmitted. As a result, linearity is not obtained immediately but onlyafter a certain time, required for the convergence of the linearizationdevice. The training of the linearization device requires the sending ofa particular sequence of data or training sequence. This remark appliesadmittedly more to adaptive predistortion than to the Cartesian loop,even if the latter requires, in order to ensure its stability, initialadjustments of phase and of amplitude levels akin to training.

The training procedure disclosed in WO 94/10765 thus relies on thetransmission by the transmitters of the system of particular sequences,so-called linearization training sequences, during linearizationtraining phases. More particularly, training sequences are transmittedin an isolated manner in time intervals forming a particular logicalchannel of the radio channels, which is dedicated solely tolinearization. However, this procedure has several drawbacks. Firstly,it requires prior synchronisation of all the transmitters so that thelatter transmit their respective linearization training sequence in thelogical channel dedicated to linearization. Moreover, no sending of datacan occur in the time intervals of this logical channel. Furthermore, atthe start of each transmission or in the event of a change of radiochannel, the transmitter is compelled to wait for the next time intervalof the logical channel dedicated to linearization, unless the system ismade considerably more complex. This is why the temporal spacing betweentwo time intervals of said logical channel cannot exceed a second, so asto guarantee a certain quality of service (QoS). This technique istherefore very prejudicial to the spectral efficiency of theradiocommunication system.

In a general manner, there exist radiocommunication systems whose framestructure is not adapted to the sending of a training sequence, forexample when no specific time interval has been provided for thispurpose when defining the frame structure.

In order to alleviate all or part of the drawbacks of the aforesaidprior art, a first aspect of the invention relates to a method oftraining a device for linearizing a radiofrequency amplifier which isincluded within a radiofrequency transmitter of a mobile terminal of aradiocommunication system comprising a fixed network and mobileterminals, which transmitter is adapted for transmitting burstsaccording to a determined frame structure, each burst comprising symbolsbelonging to a determined alphabet of symbols. The method comprises thesteps consisting in:

-   a) generating a linearization training sequence comprising a    determined number N of symbols, where N is a determined integer;-   b) transmitting the linearization training sequence by means of the    radiofrequency transmitter in at least certain of the bursts    transmitted by the latter;-   c) comparing the linearization training sequence transmitted with    the linearization training sequence generated so as to train said    linearization device.

Advantageously, in step b), the linearization training sequence isincluded in a sequence of symbols that is further designed to allow theadjusting of parameters of the transmission chain between said firstequipment and a second equipment of the radiocommunication system withwhich said first equipment communicates.

The expression transmission chain is understood to mean the set ofcomponents which participate in a bidirectional communication between afirst and a second equipments, typically a mobile terminal and the basestation with which it communicates.

Preferably, the sequence of symbols that is designed to allow theadjusting of parameters is a sequence of symbols that is designed toallow the dynamic control of the gain of a variable-gain amplifier of aradiofrequency receiver of a second equipment of the radiocommunicationsystem with which the first equipment communicates.

Stated otherwise, the training sequence is transmitted in step b) insidea time interval reserved within the frame structure for the transmissionof an AGC (Automatic Gain Control) sequence, and at the same time itensures the role of such an AGC sequence.

Thus, for the transmission of the training sequence, use is made of thetransmission time of a sequence of symbols that is necessary for otherpurposes, in this instance an AGC sequence transmitted so as to allowthe dynamic control of the transmission power of the mobile terminal atreception.

According to an advantage, the value of the symbols of the AGC sequenceis not subject to any constraint (the AGC sequence must simply be knownto the fixed network). There is therefore complete freedom to choose thesymbols of the sequence, or at least some of the symbols of thesequence, in such a way that these symbols form a satisfactory trainingsequence.

According to another advantage, the recurrence of the AGC sequence isadapted to the training requirements of the RF amplifier linearizationdevice. Specifically, the AGC sequence is in general transmitted at thestart of a frame, then upon a change of logical channel, upon a changeof RF frequency and/or upon a change of power rating. Now, it issubstantially at those moments also that there is a need for thelinearization training sequence to be transmitted.

A second aspect of the invention relates to a device for training adevice for linearizing a radiofrequency amplifier which is includedwithin a radiofrequency transmitter of a first equipment of aradiocommunication system, which transmitter is adapted for transmittingbursts according to a determined frame structure, each burst comprisingsymbols belonging to a determined alphabet of symbols. The devicecomprises:

-   a) means for generating a linearization training sequence comprising    a determined number N of symbols, where N is a determined integer;-   b) means for transmitting the linearization training sequence by    means of the transmitter in at least certain of the bursts    transmitted by the latter;-   c) means for comparing the linearization training sequence    transmitted with the linearization training sequence generated so as    to train said linearization device.

Advantageously, the linearization training sequence is included in asequence of symbols that is further designed to allow the adjusting ofparameters of the transmission chain between said first equipment and asecond equipment of the radiocommunication system with which said firstequipment communicates.

Preferably, the sequence of symbols that is designed to allow theadjusting of parameters is a sequence of symbols that is designed toallow the dynamic control of the gain of a variable-gain amplifier of aradiofrequency receiver of a second item of equipment of theradiocommunication system with which the first equipment communicates.

Stated otherwise, said means for transmitting are adapted to transmitthe training sequence inside a time interval reserved within the framestructure for the transmission of an AGC sequence, and the trainingsequence at the same time ensures the role of such an AGC sequence.

A third aspect of the invention relates to a mobile terminal of aradiocommunication system, comprising a radiofrequency transmitterhaving a radiofrequency amplifier and a device for linearizing theradiofrequency amplifier, and which further comprises a device fortraining the linearization device according to the second aspect.

A fourth aspect of the invention relates to a base station of aradiocommunication system comprising a radiofrequency transmitter havinga radiofrequency amplifier and a device for linearizing theradiofrequency amplifier, and which further comprises a device fortraining the linearization device according to the third aspect.

Other characteristics and advantages of the invention will becomefurther apparent on reading the description which follows. The latter ispurely illustrative and should be read in conjunction with the appendeddrawings in which:

FIG. 1 is a schematic diagram of an exemplary mobile terminal accordingto the invention;

FIG. 2 is a diagram illustrating a first example of bursts transmittedby the mobile terminal, without AGC sequence;

FIG. 3 is a diagram illustrating a second example of bursts transmittedby the mobile terminal, with an AGC sequence which according to theinvention comprises a linearization training sequence; and

FIG. 4 is a diagram illustrating the implementation of an AGC methodbetween a first and a second item of equipment, and vice versa.

Represented diagrammatically in FIG. 1 are the means of an exemplarymobile terminal according to the invention. Such a mobile terminalbelongs for example to a radiocommunication system which additionallycomprises a fixed network having base stations.

The terminal comprises a transmit chain 100, a receive chain 200, acontrol unit 300, a permanent memory 400, as well as an automatic gaincontrol device 500 (AGC) associated with an RF receiver of the receivechain 200.

The transmit chain 100 comprises a useful-data source 10, for example aspeech coder delivering voice-coding data. The source 10 is coupled toan M-ary data modulator 20 which provides for the baseband modulation ofthe data to be transmitted according to a modulation with M distinctstates, where M is a determined integer. The binary data which itreceives from the source 10 are translated by the modulator 20 intosymbols belonging to an M-ary alphabet, that is to say comprising Mdistinct signals. The output of modulator 20 is coupled to the input ofa radiofrequency transmitter 30. On the basis of the string of symbolsreceived, the transmitter 30 produces an RF signal suitable for radiotransmission via an antenna or a cable. The output of the transmitter 30is coupled to a transmit/receive antenna 40 via a switch 41. Thus the RFsignal produced by the transmitter is transmitted on the radio channelassociated with the transmitter.

The receive chain 200 comprises a radiofrequency receiver 50 which iscoupled to the antenna 40 via the switch 41, so as to receive an RFsignal. The receiver 50 provides for the transposition from the RFdomain to the baseband (downconversion). The receive chain 200 alsocomprises an M-ary data demodulator 60, coupled to the receiver 50. Thedata demodulator 60 provides in baseband for the demodulation of thedata of the signal received, that is to say the operation inverse tothat provided by the modulator 20. Finally, the receive chain 200comprises a data consumer device 70, such as a speech decoder, which iscoupled to the demodulator 60. This device receives as input the binarydata delivered by the demodulator 60.

The unit 300 is for example a microprocessor or a microcontroller whichprovides for the management of a mobile terminal. In particular, itcontrols the data modulator 20, the data demodulator 60, the transmitter30 and the switch 41. It also generates signaling data which aresupplied to the modulator 20 so as to be transmitted in appropriatesignaling logical channels. Conversely, the unit 300 receives from thedata demodulator 60 signaling data dispatched by the fixed network inappropriate signaling logical channels, in particular synchronizationinformation and operating commands.

The memory 400 is for example a ROM (“Read Only Memory”), EPROM(“Electrically Programmable ROM”) or Flash-EPROM memory, in which arestored data which are used for the operation of the mobile terminal.These data comprise in particular a linearization training sequence towhich we shall return later.

An exemplary embodiment of the transmitter 30 will now be described. Inthis example, the transmitter 30 comprises a radiofrequency poweramplifier 31, a radiofrequency modulator 32 which provides for thetransposition from baseband to the radiofrequency domain (upconversion),a linearization device 33, a training module 34 associated with thelinearization device.

The output of the power amplifier 31 delivers the RF signal to betransmitted. This is why this output is coupled to the antenna 40 viathe switch 41. The input of the power amplifier 31 receives aradiofrequency signal delivered by the output of the radiofrequencymodulator 32. The input of the latter is coupled to the output of thedata modulator 20 so as to receive the string of symbols forming thebaseband signal to be transmitted, through the linearization device 33.The latter comprises for example a predistortion device comprising apallet (“look-up table”) which translates each value of the signal to betransmitted into a predistorted value. As a variant or as a supplement,the device 33 can also comprise means of amplitude slaving of the outputsignal from the transmitter 30.

The training module 34 carries out the training of the linearizationdevice 33 as a function of an input signal which reflects the RF signaldelivered by the output of the power amplifier 31. For this purpose, themodule 34 receives a part of this RF signal, which part is tapped off atthe output of the power amplifier 31 by means of a coupler 36. Asneeded, the module 34 provides for the baseband return of the RF signalthus tapped off. Although being represented entirely inside thetransmitter 30, the module 34 can, at least in part, be implemented bymeans belonging to the control unit 300, in particular software means.

Finally, the automatic gain control device 500 makes it possible for thecontrol unit 300 to dynamically vary the gain of the variable gainamplifier 59 of the RF receiver 50, as a function of information whichis received from the base station with which the terminal iscommunicating, according to a method known per se. By virtue of thismethod, the base station transmits at determined instants a determinedsequence, called the AGC sequence. This sequence is known to andrecognizable by the mobile terminal. It allows the latter to measure thepower of the signal received from the base station and to deducetherefrom a control for the gain of the amplifier 59. This method isimplemented in the mobile terminal by the device 500 under the controlof the unit 300.

The principle of such a method will be described hereinbelow withreference to the diagram of FIG. 4.

The manner of operation of the mobile terminal during a phase oftraining, by the device 34, of the linearization device 33 will now bedescribed. Although it will not be mentioned each time in what follows,it is of course understood that the terms “training phase” and the terms“training sequence” refer to the training of the linearization device 33performed by the training device 34 under the control of the unit 300.

The method of training the device 33 comprises a step consisting ingenerating a training sequence comprising a determined number N ofsymbols, where N is an integer. This step is carried out by the datamodulator 20 under the control of the control unit 300. For thispurpose, the unit 300 reads a corresponding sequence of bits in thememory 400.

Next, still under the control of the unit 300, the training sequence istransmitted by means of the transmitter 30 in at least certain of thebursts transmitted by the latter, according to the frame structure ofthe system.

The training device 34 thus obtains the training sequence transmittedand compares it with the training sequence generated, and performsactions accordingly such as adaptations of predistortion coefficients orthe like of the linearization device 33, according to a specifiedtraining algorithm. This algorithm may be adaptive. One speaks ofteaching to designate these operations.

It may be noted that for any modulation it is possible to find a signalsequence of determined length N whose characteristics satisfy imposedconstraints in terms of spectral width, amplitude modulation depth,and/or others. In an example, N is equal to 10.

The AGC sequence comprises at least N symbols. It may therefore have alength greater than that of the training sequence, when it comprisesmore than N symbols. In this case, the symbols of the training sequenceare preferably the symbols of the AGC sequence which are sent first. Inthis way, the convergence of the training algorithm and there thelinearization of the RF amplifier are obtained the quickest.

Training phases may be performed periodically or in some other fashion.Other constraints may have to be taken into account after the initialtraining phase, when it is entirely suitable to correct drifting of thetransmitter. The training sequence may therefore alter both in contentand in length. The number N is therefore not necessarily fixed from onetransmission of the training sequence to another. If an increase in thesize of the sequence poses problems (for example if the frame structureis fairly inflexible), then the size N of the sequence can be fixed andjust its content can be modified as a function of the alterations in theconstraints on the system.

The diagram of FIG. 2 illustrates a first exemplary burst, which doesnot comprise any AGC sequence. In this example, the burst has a durationequal to 20 ms. It comprises firstly a ramping-up 51 of 625 μs,comprising five padding symbols, to ensure the power rise. Theexpression padding symbols is understood to mean that the binary datasent in this ramping-up are padding bits, that is to say for example astring of 0s. It next comprises a sequence of synchronization data 52whose duration is equal to around 5 ms. Next, it comprises a sequence ofuseful data 53. The useful data may be voice-coding data or moregenerally traffic data, or signaling data depending on whether the burstis transmitted on a traffic logical channel or on a signaling logicalchannel, respectively. It finally comprises a ramping-down 54, againhaving five padding symbols for the power drop. Optionally, a guard timeis moreover envisaged after the transmission of a burst, so as toguarantee the return to reception of the transmitter.

Furthermore, in any frame structure provision is made to transmitisolated bursts, in particular at each change of logical channel(occurring in particular at each turn around, that is to say switchoverfrom a receive phase to a transmit phase of the terminal), with eachchange of RF frequency (when a frequency jump functionality isimplemented by the system), with each change of transmission powerrating, or else in other particular cases that would take too long todetail here.

FIG. 3 shows an example of an isolated frame such as this comprising,before the synchronization sequence 52, an AGC sequence referenced 55.This sequence 55 is transmitted so as to allow the dynamic control, bythe fixed network, of the transmission power of the transmitter (seeabove). In this example, the sequence 52 and the sequence 55 each lastonly 1 to 3 ms. The other parts of the burst are unchanged with respectto the burst of FIG. 2. The sequence of useful data 53 may sometimes beshorter than in the case of a normal burst according to FIG. 2.

Advantageously, part of these isolated bursts is used to allow thedevice 34 for training the radiofrequency transmitter 32 to execute analgorithm for training the linearization device 33. In the example ofFIG. 3, the linearization sequence is thus included in the aforesaid AGCsequence.

It is thus possible to use the time required for the transmission of thetraining sequence for other ends such as for example the tuning of theAGC at reception, according to the method alluded to above in regard tothe diagram of FIG. 1.

The AGC sequence, and thus the training sequence, are preferablytransmitted at the start of a frame, and then upon a change of logicalchannel, upon a change of RF frequency and/or upon a change of powerrating and/or in other cases that it would be too lengthy to detailhere. This is why it is particularly advantageous to combine thesesequences (the training sequence being included within the AGCsequence).

According to another advantage, the AGC sequence is situated as near aspossible to the signal power ramping-up, for example, just after thisramping. In this way, the training of the linearization device may becarried out as quickly as possible and thus disturb transmission for theleast possible time.

It is preferable for the length of the training sequence to be such thatit does not occupy too large a portion of the burst so as to keep amaximum of symbols for the broadcasting of useful information. Thisduration obviously depends on the sought-after accuracy of the trainingalgorithm but a compromise between accuracy and duration often turns outto be necessary in order to preserve a maximum of useful information inthe burst. A reasonable compromise is achieved when it represents around5% of the total duration of the burst. In the case of a 20 ms bursttransmitted at a binary rate of 8 ksymbols/s, the duration of a trainingsequence of N=10 symbols is thus equal to 1.25 ms, i.e. 6.25% of thetotal duration of the frame.

The diagram of FIG. 4 illustrates the implementation of an AGC method(known per se) between a first item of equipment 5 and a second item ofequipment 5′ of a radiocommunication system.

The item of equipment 5 is here a mobile terminal for example such asdescribed above in regard to FIG. 1. It comprises the RF transmitter 30and the RF receiver 50, the latter comprising the variable-gainamplifier 59. The item of equipment 5′ is here a base station with whichthe mobile terminal 5 communicates, which comprises an RF transmitter30′ and an RF receiver 50′ having a variable-gain amplifier 59′.Operationally, the components 30′, 50′ and 59′ of the base station 5′are identical or comparable to the components 30, 50 and 59 respectivelyof the mobile terminal 5′. These components are not detailed again here.

An AGC sequence transmitted by the mobile terminal 5 allows the dynamiccontrol of the gain of the amplifier 59′ of the receiver 50′ of the basestation 5′. Conversely, an AGC sequence transmitted by the base station5′ allows the dynamic control of the gain of the amplifier 59 of thereceiver 50 of the mobile terminal.

1. A method of training a device for linearizing a radiofrequencyamplifier which is included within a radiofrequency transmitter of afirst equipment of a radiocommunication system, which transmitter isadapted for transmitting bursts according to a determined framestructure, each burst comprising symbols belonging to a determinedalphabet of symbols, the method comprising the steps consisting in: a)generating a linearization training sequence comprising a determinednumber N of symbols, where N is a determined integer; b) transmittingthe linearization training sequence by means of the transmitter in atleast certain of the bursts transmitted by the latter; c) comparing thelinearization training sequence transmitted with the linearizationtraining sequence generated so as to teach said linearization device,wherein, in step b), the linearization training sequence is included ina sequence of symbols that is further designed to allow the adjusting ofparameters of the transmission chain between said first equipment and asecond equipment of the radiocommunication system with which said firstequipment communicates.
 2. The method of claim 1, wherein the sequenceof symbols that is designed to allow the adjusting of parameters is asequence of symbols that is designed to allow the dynamic control of thegain of a variable-gain amplifier of a radiofrequency receiver of asecond equipment of the radiocommunication system with which the firstequipment communicates.
 3. The method of claim 1, wherein thelinearization training sequence occupies only a part of the burst inwhich it is transmitted.
 4. The method of claim 3, wherein thelinearization training sequence occupies around 5% of the duration ofthe burst in which it is transmitted.
 5. The method of claim 1, whereinthe linearization training sequence is transmitted at the start of theframe.
 6. The method of claim 1, wherein the linearization trainingsequence is further transmitted during a change of logical channel, achange of frequency and/or a change of power rating of the firstequipment.
 7. The method of claim 1, wherein the sequence of symbolsthat is designed to allow the dynamic control of the transmission powerof the first equipment comprises more than N symbols, and according towhich said N symbols of the linearization training sequence are thesymbols of the sequence of symbols that is designed to allow the dynamiccontrol of the transmission power of the first equipment which are sentfirst.
 8. A device for training a device for linearizing aradiofrequency amplifier which is included within a radiofrequencytransmitter of a first equipment of a radiocommunication system, whichtransmitter is adapted for transmitting bursts according to a determinedframe structure, each burst comprising symbols belonging to a determinedalphabet of symbols, the device comprising: a) means for generating alinearization training sequence comprising a determined number N ofsymbols, where N is a determined integer; b) means for transmitting thelinearization training sequence by means of the transmitter in at leastcertain of the bursts transmitted by the latter; c) means for comparingthe linearization training sequence transmitted with the linearizationtraining sequence generated so as to teach said linearization device,wherein said linearization training sequence is included in a sequenceof symbols that is further designed to allow the adjusting of parametersof the transmission chain between said first equipment and a secondequipment of the radiocommunication system with which said first item ofequipment communicates.
 9. The device of claim 8, wherein the sequenceof symbols that is designed to allow the adjusting of parameters is asequence of symbols that is designed to allow the dynamic control of thegain of a variable-gain amplifier of a radiofrequency receiver of asecond equipment of the radiocommunication system with which the firstequipment communicates.
 10. The device of claim 8, wherein thelinearization training sequence occupies only a part of the burst inwhich it is transmitted.
 11. The device of claim 10, wherein thelinearization training sequence occupies around 5% of the duration ofthe burst in which it is transmitted.
 12. The device of claim 8, whereinthe means for transmitting are adapted for transmitting thelinearization training sequence at the start of the frame.
 13. Thedevice of claim 8, wherein the means for transmitting are adapted fortransmitting, moreover, the linearization training sequence during achange of logical channel, a change of frequency and/or a change ofpower rating of the first equipment.
 14. The device of claim 8, whereinthe sequence of symbols that is designed to allow the adjusting ofparameters comprises more than N symbols, and wherein said N symbols ofthe linearization training sequence are the symbols of the sequence ofsymbols that is designed to allow the adjusting of parameters which aresent first.
 15. A mobile terminal of a radiocommunication system,comprising a radiofrequency transmitter having a radiofrequencyamplifier and a device for linearizing the radiofrequency amplifier,further comprising a device for training the linearization device asclaimed in claim
 8. 16. A base station of a radiocommunication systemcomprising a radiofrequency transmitter having a radiofrequencyamplifier and a device for linearizing the radiofrequency amplifier,further comprising a device for training the linearization device asclaimed in claim 8.